The present invention relates to a thermal regenerative compressive device and particularly to a compressive device that employs an adsorption cycle. Adsorption cycles can be used in heat-driven refrigerators, air conditioning and in heat pumps in which the energy source is a burning fuel or waste heat.
The conventional vapour compression cycle demands a mechanical work input that is normally provided via an electric motor. In comparison with such conventional vapour compression machines, high efficiency adsorption machines can provide a reduction in running costs, the use of primary energy for cooling or heating and an associated reduction in CO2 emissions. This latter advantage is particularly important in the light of growing concerns over the xe2x80x98greenhouse effectxe2x80x99. Even larger savings can be made where the waste heat is used as the driving energy in a regenerative cycle.
Adsorption or absorption refrigeration and heat pump cycles rely on the adsorption or absorption of a refrigerant gas, such as ammonia, into a solid adsorbent or solid/liquid absorbent at low pressure and subsequent desorption by heating. The sorbent acts as a xe2x80x98chemical compressorxe2x80x99 driven by heat. A brief description of a simple adsorption cycle is given below to assist in an understanding of this cycle that is central to the operation of the compressive device of the present invention.
In its simplest form an adsorption refrigerator consists of two linked vessels 2,4. The first vessel 2 contains adsorbent 3 and both vessels contain refrigerant as shown in FIG. 1. Initially, as shown in FIG. 1a the whole assembly is at low pressure and temperature. The adsorbent 3 contains a large concentration of refrigerant within it and the second vessel 4 contains refrigerant gas. The adsorbent vessel 2, otherwise known as the generator, is then heated driving out the refrigerant and raising the system pressure. The desorbed refrigerant condenses as a liquid in the second vessel 4 and the process rejects heat to the environment surrounding the vessel (FIG. 1b). The heat rejected by the second vessel 4 is part of the useful heat output of a heat pump. The generator 2 is then cooled back to ambient temperature re-adsorbing the refrigerant and reducing the system pressure. The reduced pressure above the liquid in the second vessel 4 causes the liquid to boil (FIG. 1c). Heat is absorbed whilst the liquid boils, producing a cooling/refrigeration effect to the environment surrounding the second vessel 4. The heat drawn from the generator 2 forms the other part of the useful heat output of a heat pump.
It should be noted that the cycle described above is discontinuous since useful cooling only occurs for half of the cycle. However, two such systems operated out of phase could theoretically provide continuous cooling. This basic arrangement of a heat-driven adsorbent compressive device has a comparatively low Coefficient Of Performance (Refrigeration COP=Cooling/Heat Input and Heat Pump COP=Heat Output/Heat Input). Also, as the thermal conductivity of the adsorbent bed in the generator 2 is generally poor, the time taken for a cycle could be an hour or more and the cooling power per mass of adsorbent could be less than 100 W/kg. Improved heat transfer can reduce this cycle time to a few minutes which in turn increases the power density of the adsorbent to the order of 1 kW/kg. Where two or more adsorbent beds are used out of phase with one another, heat from one of the beds can be re-used to pre-heat another of the adsorbent beds, i.e. the heat can be xe2x80x98regeneratedxe2x80x99. This can improve the COP as the heat rejected by one adsorbent bed can provide a large part of the heat required for desorption in another bed.
Despite considerable research in this area, however, only two established types of heat driven air conditioners are currently on the market. Lithium bromidexe2x80x94water air conditioners can be very efficient but are unable to provide cooling temperatures  less than 0xc2x0 C. and only become viable in  greater than 100 kW sizes. Also, such air conditioners can only be used in combination with a cooling tower. Smaller (15 kW) ammoniaxe2x80x94water chillers have very poor efficiency.
In U.S. Pat. No. 5,503,222 a carousel heat exchange is described in which a plurality of heater tubes are arranged radially around a rotational axis. Each heater tube contains a solid adsorbent and a refrigerant and is divided into two zones. Radially extending baffles define a series of axially extending temperate sections through which a heat carrier fluid, such as air, flows across the heater tubes in an axial direction. Rotation of the heat exchanger results in the heater tubes crossing each of the temperate sections in turn.
U.S. Pat. No. 4,660,629 describes a similar heat-driven adsorption device again consisting of a plurality of adsorption chambers arranged radially about a rotational axis and intersecting an axial flow of heat carrier fluid.
The heat-driven adsorption devices described in the above mentioned documents have complex structures involving a plurality of separated axial heat carrier fluid streams across which the radially arranged adsorption containers pass. That is to say the heat carrier fluid is in cross-flow with respect to the movement of the adsorption containers. Also, the structure of the adsorption devices described in the documents referred to above prevents or limits the opportunity to regenerate heat and so establishes an upper limit with regard to the efficiency of the devices.
The present invention, on the other hand, seeks to provide a compressive device, that employs an adsorption cycle, that enables regeneration of heat in a simple and effective manner and so is capable of achieving greater efficiencies than previous adsorption devices.
The present invention provides a compressive device comprising: a plurality of sorbent vessels each vessel containing a sorbent material and a sorbate fluid; one or more fluid conduits in which at least a portion of each of the plurality of sorbent vessels is located and in which a heat carrier fluid flows, the conduit having an inlet and an outlet; and driving means for causing relative cyclical movement of the sorbent vessels with respect to the inlet of the conduit such that the heater carrier fluid at the inlet is in counterflow with respect to the sorbent vessels.
In a preferred embodiment, the conduit is stationary and the driving means is connected to the plurality of sorbent vessels. The conduit may be a cylindrical annulus within which the sorbent vessels move with the axis of the sorbent vessels aligned with the central axis of the cylinder. In this way the heat carrier fluid is in counterflow with respect to the rotational movement of the sorbent vessels throughout the conduit. The sorbent vessels may be mounted about an edge region of a rotating disc with the rotating disc forming a sliding fluid seal with respect to the conduit.
Ideally, a heating device for heating the heat carrier fluid is provided at an intermediate position between the inlet and the outlet to the conduit. The heating device may be positioned away from the conduit with a passageway fluidly connecting the conduit to the heating device. Alternatively, the heating device may be positioned so as to provide direct heating of the carrier fluid within the conduit or the heating device may consist of a valve for the introduction of a heated fluid, additional to the heat carrier fluid, into the conduit.
Moreover, a seal is preferably provided in the conduit between the inlet and the outlet to direct the flow of the heat carrier fluid in a selected path through the conduit. The fluid seal may consist of a plurality of vanes connected to a rotating axle wherein the end of each vane, distant from the axle, forms a sliding fluid seal with respect the conduit. Alternatively, the fluid seal may be in the form of a constriction in the conduit.
A plurality of heating/cooling channels may be provided and each of the sorbent vessels has a second portion, distant from the first portion, said driving means causing relative cyclical movement of the second portions of the sorbent vessels with respect to the heating/cooling channels. Preferably, the heating/cooling channels are arranged to produce a flow of fluid over the second portions of each of the sorbent vessels.
With the present invention a continuous and steady output of heating/cooling can be achieved. Also, with appropriate adsorbent material a high power density in excess of 1 kW cooling/kg of adsorbent can be achieved and a system volume of only 0.2 m3 for a 10 kW chiller. Moreover, the absence of specialist ammonia valves, pumps and control equipment greatly simplifies the construction and maintenance of the compressive device and reduces the overall cost of the device. For a gas-fired air conditioning system it is believed that a COP of 0.95 is achievable with the present invention due to the regenerative nature of the cycle employed.
Preferably the absorbent material is an active carbon. However, alternative material may be used such as zeolites, silica gels, metal hydrides, chemical absorbents such as calcium chloride and liquid absorbents such as water or lithium bromide.
The compressive device of the present invention may be used in direct air-conditioning or refrigeration. The present invention may additionally be employed in the chilling of liquids or in ice making. Indeed, the compressive device of the present invention is suitable in all cases where heating/cooling is required including air conditioning, industrial refrigeration, in heat pumps or as a thermal transformer.
Although reference is generally made herein to solid adsorbents it is to be understood that the present invention may alternatively employ fluid absorbents such as water with ammonia as the refrigerant, for example.